The present invention relates to the use of fatty acid salts to create steam foams in conjunction with steam floods to improve conformance.
When an oil reservoir is subjected to steam injection, steam tends to move up in the formation, whereas condensate and oil tends to move down due to the density difference between the fluids. Gradually, a steam override condition develops, in which the injected steam sweeps the upper portion of the formation but leaves the lower portion untouched. Injected steam will tend to follow the path of least resistance from the injection well to a production well. Thus, areas of high permeability will receive more and more of the injected steam which further raises the permeability of such areas. This phenomenon exists to an even larger degree with low injection rates and thick formations. The steam override problem worsens at greater radial distances from the injection well because steam flux decreases with increasing steam zone radius.
Although residual oil saturation in the steam swept region can be as low as 10% the average residual oil saturation in the formation remains much higher due to poor vertical conformance. Thus it is because of the creation of steam override zones that vertical conformance in steam floods is usually poor.
It has long been the concern of the oil industry to improve the conformance of a steam flood by reducing the permeability of the steam swept zone by various means. The injection of numerous chemicals such as foams, foaming solutions, gelling solutions or plugging or precipitating solutions have been tried. Because of the danger of damaging the reservoir, it is considered important to have a non-permanent means of lowering permeability in the steam override zones. For this reason, certain plugging agents are deemed not acceptable. In order to successfully divert steam and improve vertical conformance, the injected chemical should be (1) stable at high steam temperatures of about 300.degree. to about 600.degree. F., (2) effective in reducing permeability in steam swept zones, (3) non-damaging to the oil reservoir and (4) economical.
The literature is replete with references to various foaming agents which are employed to lower permeability in steam swept zones. The vast majority of foaming agents in the prior art require the injection of a non-condensable gas to generate the foam in conjunction with the injection of steam and the foaming agent. Copending U.S. patent application Ser. No. 07/896,710, filed Jun. 10, 1992, now U.S. Pat. No. 5,279,367, discloses the use of fatty acids having about 12 to 20 carbon atoms as additives to surfactant foaming solutions used in steam or carbon dioxide floods where the ratio of fatty acid to surfactant in the foaming solution is between about 1:4 and 3:2.